1. Field of the Invention
The present invention relates to a current collecting electrode of a photovoltaic device which has excellent initial characteristics, and high humidity resistance and high reliability throughout a long use term.
2. Related Background Art
In conventional photovoltaic devices, an electrode is formed in a comb-shape pattern so as not to intercept incident light, and to keep the series resistance low. The electrode is formed from a metal exhibiting satisfactory ohmic contact to Si by direct deposition of the metal on Si by vacuum deposition or by a sputtering method, or deposition by an electroless plating method.
Such methods, however, require a vacuum process, or a mask for patterning, and take a long time, which is disadvantageous in mass-production. In an alternative process, an electroconductive resin is applied in an electrode shape by screen printing and is baked as an electrode. This process is suitable for mass-production because of the high throughput.
In the solar cell for general home use, for example, when an output of about 3 kW is required and the conversion efficiency is 10%, the cell requires a large area, as large as 30 m.sup.2. Such a large solar cell requires a busbar for collecting the current from the electrode, which needs additional manufacturing processing.
The larger solar cell has necessarily more defects, and the defects will cause short-circuiting or shunting, which lowers significantly the conversion efficiency. The defects which are apart from the electrode or the busbar cause less current loss owing to the resistance against the current flowing into the defective portions, whereas the defects which are under the electrode or the busbar cause larger current losses.
To solve the problems in collecting current from the electrodes formed by the screen printing as described above by using the busbar, and to form electrodes suitable for a solar cell of a large area, U.S. Pat. No. 4,260,429 discloses a method in which a copper wire is coated with a solid polymer containing electroconductive particles to form an electrode and the electrode is attached on a photoelectric cell; and U.S. Pat. No. 5,084,107 discloses a method in which a metal wire is fixed via an electroconductive adhesive on an electrode position on a photovoltaic device. Such an electrode constituted of an electroconductive wire coated with a solid polymer containing electroconductive particles (electroconductive adhesive) can keep the ohmic loss low even with an electrode of longer than 10 cm effectively.
The above prior art techniques involve the following problems however:
In U.S. Pat. No. 4,260,429, no special treatment is given to the surface of the electroconductive wire or of the metallic member. Generally, when an electrode is prepared from an electroconductive wire or a metal of readily oxidizable material such as copper without surface treatment according to the disclosed method, the surface of the conductive wire or the metal will be oxidized, not giving sufficient adhesion at the interface between the surface of the conductive wire and the solid polymer containing electroconductive particles or the electroconductive adhesive. Further, the surface oxidation proceeds under the influence of moisture to weaken the adhesion.
Moreover, without special treatment of the electroconductive wire or metallic member, initial power loss is caused in the solar cell, and separation occurs at the adhesion interface to increase the series resistance and lower the conversion efficiency during a long use term, thus impairing the reliability.
In U.S. Pat. No. 5,084,107, the wire material of a metal such as copper, or of an alloy of mainly composed of copper is plated with another metal or alloy. Usually, the plating of the wire is conducted batch-wise with the metal wire cut in a predetermined length, so that the wire is liable to be curved undesirably or to cause defective plating because of the batch plating treatment of the cut wire. The batch treatment increases the number of the production steps, disadvantageously. In the film formation by plating, the plating conditions such as the electric current density, the bath concentration, the bath temperature, and the bath stirring should be strictly adjusted to control the crystal nucleus formation and the crystal growth rate. A larger diameter of the crystal particle in the film causes diffusion of the metal on the inside of the film by heat or moisture. When the metal members are not completely coated with the resin, adhesiveness to the photovoltaic device varies, causing problems of electromigration or the like by the metal ions of the metal members due to the influence of moisture.